1. Field of the Invention
This invention relates to a stencil printer, and more particularly to paper feed control in a stencil printer in which printing papers are fed between a stencil master wrapped around a printing drum and a press roller.
2. Description of the Related Art
In a stencil printer, a stencil master is wrapped around a printing drum and the printing drum is rotated. A press roller in contact with the stencil master on the printing drum is rotated together with the printing drum and a printing paper is fed between the stencil master and the press roller by a paper feed mechanism. The printing paper is conveyed pinched between the stencil master and the press roller and ink supplied inside the printing drum is transferred to the printing paper through perforations in the stencil master.
In such a stencil printer, the printing paper must be fed between the printing drum and the press roller at a timing such that the printing paper exactly overlaps with the stencil master in a predetermined position relative to the stencil master. For this purpose, the paper feed mechanism is arranged to be constantly driven with a predetermined phase difference or a predetermined ratio of speeds relative to the printing drum, and upon starting of printing, adjustment for ensuring that the printing paper exactly overlaps with the stencil master in a predetermined position is carried out.
In conventional stencil printers, the paper feed mechanism generally comprises primary and secondary paper feed sections which are driven by the printing drum by way of a transmission mechanism such as those including gears.
The primary and secondary paper feed sections in the conventional stencil printers will be described hereinbelow.
In the primary paper feed section, printing papers stacked on a paper feed table are fed one by one for one rotation of the printing drum by a pickup roller and scraper and conveyed to the secondary paper feed section. The pickup roller and the scraper are intermittently rotated by a main motor, which drives the printing drum, by way of a paper feed clutch which is selectively engaged and disengaged on the basis of a signal from a drum position sensor which detects the angular position of the printing drum. The pickup roller and the scraper are provided with a one-way clutch and the paper feed clutch is disengaged after the primary paper feed section delivers the leading end of the printing paper to the secondary paper feed section so that the pickup roller and the scraper run free and back tension is reduced.
In the secondary paper feed section, the leading end of the printing paper fed by the pickup roller and the scraper abuts against a guide roller or a timing roller near the contact line of the guide roller and the timing roller (will be referred to as "the conveyor roller pair", hereinbelow) which are stopped and the printing paper sags. Then the conveyor roller pair are started when the printing drum is in a predetermined phase of rotation. Each roller of the conveyor roller pair is provided with a gear on each end of its shaft and the gears on the shafts of the rollers on each end thereof are in mesh with each other. The guide roller is caused to make several rotations in one direction per one rotation of the printing drum by the main motor by way of a transmission mechanism comprising gears or an endless belt, a cam, a sector gear, a one-way clutch and the like. The timing roller is rotated in the direction opposite to the guide roller driven by the guide roller. The timing roller is moved away from the guide roller after the guide roller is stopped by a mechanism including, for instance, a cam, a cam follower, a link member and a resilient member. Further, the timing roller is provided with a spring or an electromagnetic brake on one end of its shaft so that the timing roller is stopped as soon as it is disengaged from the guide roller without overshooting under inertia.
The printing paper conveyed by the conveyor roller pair is fed between the printing drum and the press roller pressed against the printing drum at a predetermined pressure and ink supplied from an ink supply section disposed inside the printing drum is transferred to the printing paper through image-wise perforations in the stencil master while the printing paper is conveyed pinched by the printing drum and the press roller.
A clamp mechanism which clamps an end of the stencil master and holds the stencil master on the circumferential surface of the printing drum is provided on the circumferential surface of the printing drum. Since the clamp mechanism projects outward in a radial direction of the printing drum, the press roller is moved between an operative position where it is in contact with the printing drum and a retracted position where it is away from the printing drum in order to prevent the clamping mechanism from interfering with the press roller.
The position of the printing paper relative to the image region of the stencil master in the direction of feed of the printing paper is adjusted (this adjustment will be referred to as "longitudinal registration", hereinbelow) by changing the timing at which the conveyor roller pair start conveying the printing paper by changing the rotation phase of the cam, which governs starting the guide roller of the conveyor roller pair, relative to the rotation phase of the printing drum.
However in the conventional paper feed mechanism, fluctuation in rotating speed of the main motor itself and fluctuation in rotating speed of the printing drum due to external factors such as impact when the press roller is brought into contact with the printing drum can occur. Further, phase shift between the printing drum and the conveyor roller pair can be generated due to backlash in the transmission mechanism including gears, an endless belt and the like which transmits torque of the main motor to the printing drum and the conveyor roller pair. Further since the printing drum and the conveyor roller pair are driven by the same main motor, it is almost impossible to control rotation of the conveyor roller pair to compensate for fluctuation in the rotating speed of the printing drum. Accordingly it is difficult to exactly register the leading end of the printing paper to a predetermined position on the stencil master on the printing drum and it is also difficult to adjust the mechanism for such registration. Further there has been fear that the mechanism for the longitudinal registration increases the phase shift between the printing drum and the conveyor roller pair due to said backlash. Thus there has been a problem that the printing paper is shifted from the desired position relative to the stencil master. This problem will be referred to as "position shift of the printing paper", hereinbelow.
Such a position shift of the printing paper can be generated also due to slip of the printing paper on the circumferential surfaces of the conveyor roller pair. That is, the printing paper can slip on the circumferential surfaces of the rollers due to paper dust thereon and/or wear of the rollers, which results in an amount of feed of the printing paper smaller than that expected. The difference between the actual amount of feed of the printing paper and the expected amount of feed of the printing paper cannot be compensated for and results in the position shift of the printing paper.
Further conventionally the stencil master is positioned relative to the clamp mechanism on the printing drum by pulse control of the stepping motor which drives the conveyor roller for conveying the stencil master. However when the leading end portion of the stencil master is curved or the stencil master slips on the conveyor roller, it is difficult to accurately position the stencil master in a desired position relative to the clamp mechanism, which also results in the position shift of the printing paper.
Further since the clamp mechanism generally holds the leading end portion of the stencil master, the stencil master can shift relative to the printing drum in the direction opposite to the direction of rotation of the printing drum due to repetition of actions of bringing the press roller into contact with and away from the printing drum and/or tensile force momentarily generated when the trailing end portion of the printing paper is caught by the pickup roller and the scraper. Such shift of the stencil master also results in the position shift of the printing paper even if the actual amount of feed of the printing paper is equal to the expected amount of feed of the printing paper.